The invention concerns a method for heteronuclear decoupling in fast magic-angle spinning nuclear magnetic resonance measurements of solid samples.
Efficient heteronuclear decoupling is one of the most challenging requirements that must be fulfilled to obtain high-resolution NMR spectra of solids. In powders containing directly bonded dilute spins S=13C and abundant spins I=1H, efficient proton decoupling requires a reduction of the heteronuclear dipolar interactions by no less than three orders of magnitude. In polycrystalline or amorphous samples studied by magic-angle spinning (MAS) with slow spinning frequencies νrot of a few kHz, continuous-wave (CW) irradiation of the abundant I spins with an RF amplitude (also known as nutation or Rabi frequency ν1I=−γB1) in the range 60<ν1I<80 kHz remains the simplest way to achieve efficient heteronuclear decoupling. At higher spinning speeds, more elaborate schemes have been proposed that use π phase shifted (XiX) [1,2] or two-pulse phase-modulated (TPPM) techniques [3]. The success of these methods has lead to renewed interest in the intricate mechanisms underlying efficient decoupling in rotating solids [4]. Several variants of TPPM [5-8] and more sophisticated decoupling schemes have been developed [9-11].
Recent progress in the design of MAS probes and in superconducting magnet technology, fuelled by the apparently unquenchable demand for enhanced sensitivity and spectral resolution, opens the way to very fast spinning frequencies and to ever-increasing static fields. Higher spinning frequencies lead to a more effective averaging of dipolar interactions. It may be useful to recouple dipolar interactions when they give access to structural information. Many recent methodological developments deliberately exploit recoupling to drive the transfer of magnetization from one spin to another. Yet, recoupling can also occur unwittingly, so that the efficiency of heteronuclear decoupling is compromised. Once identified, it is possible to combat these deleterious effects by suitable phase-modulated schemes.
New recoupling schemes require the design of appropriately tailored RF pulse sequences [12]. One of the simplest methods to recouple dipolar interactions in rotating solids is known as rotary resonance recoupling (R3) [13]. It consists in applying an unmodulated continuous-wave (CW) RF field with an amplitude ν1 adjusted to fulfil one of the conditions ν1=nνrot with n=½, 1 and 2 [13-14]. Besides the recovery of the chemical shift anisotropy (CSA) at n=1 and 2, the recoupling of homonuclear interactions can occur at n=½ and 1, while heteronuclear interactions are recoupled for n=1, 2, 3, . . . . Recoupling by rotary resonance has been exploited both for structural studies and to induce a transfer of magnetization [12-19].
However, rotary resonance can also manifest itself through a partial breakdown of the efficiency of heteronuclear dipolar decoupling [20, 21], leading to a broadening of the resonances of dilute spins S. To avoid this undesirable broadening, the nutation frequency should be at least 4 times higher than the spinning frequency, i.e., ν1I>4νrot [8, 22]. Clearly, with increasing spinning speeds (up to 70 kHz nowadays), it becomes more and more challenging to avoid rotary resonance interferences. Yet, surprisingly, rotary resonance-induced line broadening seems to be perceived so far as inevitable collateral damage of the combination of decoupling and spinning.
[30] discloses a broadband decoupling sequence designed for liquid crystals which is based on a phase modulation of pulses with phase shifts through small angles.
[31] refers to a recoupling method and discloses a magic angle spinning experiment for separating undistorted quasi-static chemical shift powder patterns, using only 360° pulses with suitable phases which makes the spectral patterns insensitive to pulse length errors and other imperfections.
Object of the invention is to present a method that allows one to quench the above described type of interference to a large extent.